Optical compensators are used in prior art to improve the optical properties of liquid crystal displays (LCD), such as the contrast ratio and the grey scale representation at large viewing angles. For example in uncompensated displays of the TN or STN type at large viewing angles often a change of the grey levels and even grey scale inversion, as well as a loss of contrast and undesired changes of the colour gamut are observed.
An overview of the LCD technology and the principles and methods of optical compensation of LCDs is given in U.S. Pat. No. 5,619,352, the entire disclosure of which is incorporated into this application by way of reference. As described in U.S. Pat. No. 5,619,352, to improve the contrast of a display at wide viewing angles a negatively birefringent C-plate compensator can be used, however, such a compensator does not improve the greyscale representation of the display. On the other hand, to suppress or even eliminate grey scale inversion and improve the grey scale stability U.S. Pat. No. 5,619,352 suggests to use a birefringent O-plate compensator. An O-plate compensator as described in U.S. Pat. No. 5,619,352 includes an O-plate, and may additionally include one or more A-plates and/or negative C-plates.
The terms ‘O-plate’, ‘A-plate’ and ‘C-plate’ as used in U.S. Pat. No. 5,619,352 and throughout this invention have the following meanings. An ‘O-plate’ is an optical retarder utilizing a layer of a positively birefringent (e.g. liquid crystal) material with its principal optical axis oriented at an oblique angle with respect to the plane of the layer. An ‘A-plate’ is an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer, and its ordinary axis (also called ‘a-axis’) oriented perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light. A ‘C-plate’ is an optical retarder utilizing a layer of a uniaxially birefringent material with its extraordinary axis (also called ‘c-axis’) perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light.
Negative birefringent C-plate retarders in prior art have been prepared for example from uniaxially compressed films of isotropic polymers, by vapour deposition of inorganic thin films, as described for example in U.S. Pat. No. 5,196,953, or from negatively birefringent liquid crystal materials. However, stretched or compressed polymer films often show only moderate birefringence and require high film thickness, vapour deposition requires complicated manufacturing procedures, and negatively birefringent liquid crystal materials are often less easily available and more expensive than positively birefringent materials.
To overcome these disadvantages, it has recently been suggested, for example in WO 01/20393 and WO 01/20394, to use a cholesteric liquid crystal film with short pitch, typically with its Bragg reflection band in the UV region of the electromagnetic spectrum. Such a film exhibits negative birefringent C-type retardation for wavelengths greater than its reflection maximum. The refractive index ellipsoid of this type of film approximates to that of a vertically aligned liquid crystal with negative birefringence. Such a retardation film can be used for example to cancel off-axis retardation in the homeotropically driven dark state of a TN-LCD, and thus significantly improve the viewing angle of the LC display.
WO 01/20393 discloses a compensator that is a combination of a planar A-plate, an O-plate and a negative C-plate, wherein the negative C-plate comprises a short-pitch cholesteric LC film. When used for example in a TN-LCD, this combination provides excellent contrast at horizontal viewing angles and reduces unwanted changes of the colour gamut. However, its performance at vertical viewing angles is limited. Furthermore, the use of multiple retardation films is expensive and raises manufacturing and durability problems.
One aim of the present invention is to provide an optical compensator which has improved performance for compensation of LCDs, is easy to manufacture, in particularly for mass production, and does not have the drawbacks of prior art compensators as described above. Another aim of the invention is to provide an improved method for its preparation. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors have found that the above described problems can be solved, and an optical compensator with superior performance can be obtained, by combining multiple films in a single layer and by using a biaxial C-plate retarder. It was found that a biaxial negative C-plate retarder in its optical properties approximates to a combination of a planar A-plate and a negative C-plate, but shows better optical performance than such a combination. The in-plane anisotropy of the biaxial negative C-plate retarder (Δnxy) approximates to the A-plate and the out-of-plane anisotropy (Δnxz and Δnyz) to the negative C-plate. Simulations have shown that the optical performance of the biaxial negative C-plate retarder is surprisingly superior to that of the A-plate and negative C-plate stacked sequentially, and shows exceptionally good viewing-angle performance for liquid crystal displays. Furthermore, the use of a single biaxial film instead of two stacked films reduces costs and manufacturing problems.
Moreover, the inventors have found a new and improved method of preparing such a film, by using a polymerisable chiral liquid crystal material comprising a photosensitive compound, and irradiating the material with polarised UV light, leading to a change of shape or structure and thus a birefringence change of the photosensitive compounds in some parts of the film, which is then frozen in by polymerisation.
WO 03/01544 discloses a cholesteric biaxial film having a deformed helix structure with an elliptical refractive index and a method of preparing such a film from a polymerisable cholesteric material comprising a dichroic photoinitiator. However, it does not disclose a biaxial film with uniform helical structure and varying birefringence obtained by the methods and from materials comprising a photosensitive compound as claimed in the present invention.
U.S. Pat. No. 6,685,998 discloses a compensator comprising a transparent substrate and a film formed from rod-like liquid crystal molecules, wherein said film has three different principal refractive indices and the liquid crystal molecules are oriented in cholesteric alignment. However, it does not disclose a biaxial cholesteric film reflecting in the UV range and having a periodically varying birefringence obtained by the methods and from the materials as claimed in the present invention.